Method and Apparatus for Radiation Detection in a High Temperature Environment

ABSTRACT

A radiation detector operating at high temperatures is shown comprising a scintillating material for producing light when excited by incident radiation, a photocathode, and an electron multiplier. The photocathode is deposited directly onto the surface of the scintillating material that is oriented toward the electron multiplier. Depositing the photocathode directly on the surface greatly decreases photon loss which is a problem of prior art systems. In a preferred embodiment, a metal flange is hermetically sealed to the scintillating material and this is fusion welded to the electron multiplier to create a vacuum envelope. This invention is particularly useful in noisy environments such as downhole in a drilling operatio

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for radiationdetection. More specifically, this invention relates to a method andapparatus for radiation detection in a high temperature environment.

The detection of radiation through the use of scintillating material hasbeen successfully employed in the past in a variety of contexts. Inthis, light is produced by a scintillating material in response toincident radiation. This light is operably converted to an electricalsignal through the use of a photomultiplier (“PMT”) and this electricalsignal can be used to determine the amount and strength of the incidentradiation.

In conventional radiation detectors utilizing scintillating material,the scintillating material and PMT are usually two separate components.Each is fabricated independently and they are brought together at afinal stage of construction. The common practice in creating this typeof detector is to first cut and polish the scintillating material to therequired shape and then encapsulate it within a light-tight,mechanically protective container with optically reflective material onall sides except for one of the end faces to allow the scintillatorlight to be transmitted to the PMT cathode. An optical window is thenattached to this open end through the use of a coupling agent such assilicone. The clear face of the scintillating material is then coupledwith a PMT using another coupling agent.

One problem with conventional scintillation-type radiation detectors isthe fact that at each of the couplings, photons created in thescintillation process are abated because of a mismatch in the indices ofrefraction or because of absorption.

Prior attempts to remedy the problem of photon loss have primarily beendirected to decreasing the size of the gaps between the scintillator,the window, and the PMT. Work has also been envisioned in the selectionof the type of window into the PMT in hopes of transmitting the maximumamount of light from the scintillating material. Other priorconstructions have deposited the photocathode directly on thescintillating material.

All of these approaches suffered from one or more limitations. While thedecrease in the size of the interface gaps is helpful, it is notpossible to eliminate gaps entirely. As such, photons continue to belost in traversing gaps and coupling agents. Window design entailscompromises and direct deposit applications have not been useful in hightemperature environments.

Scintillation type radiation detectors find useful application in avariety of environments. One application of particular interest in thepetroleum industry is the use of scintillation technology to makedensity and other measurements within a well hole. Downholeapplications, however, present a challenge because of the temperature atwhich the radiation detector must operate and the fact that increasedvibration may affect the integrity of the interfaces between components

The difficulties and limitations suggested in the preceding are notintended to be exhaustive, but rather are among many which demonstratethat although significant attention has been devoted to increasingefficiency and sensitivity in scintillating type radiation detectors,the prior attempts do not satisfy both the desired increase inefficiency and the need for operation in noisy, high temperatureenvironments.

BRIEF SUMMARY OF A PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of the invention comprises a method and apparatusfor measuring radiation in a high temperature environment. In apreferred embodiment, a photocathode is deposited directly onto aconcave surface of a scintillating material. This surface is orientedtoward an electron multiplier comprising focus electrodes and a dynodestack. Radiation incident on the scintillating material causes it togenerate light, when this light comes into contact with thephotocathode, electrons are output that are then amplified by theelectron multiplier and output on a wire for measurement.

THE DRAWINGS

Objects and advantages of the present invention will become apparentfrom the following detailed description of embodiments taken inconjunction with the accompanying drawings, wherein:

FIG. 1A is a schematic view of the context in which the presentinvention can be used to advantage.

FIG. 1B is an expanded schematic view of a scintillator system formeasuring density of fluid material within a borehole.

FIG. 2 is a cross-sectional schematic view of one preferred embodimentof the present invention where the scintillating material is an integralpart of a vacuum envelope.

FIG. 3 is a cross-sectional schematic view of another preferredembodiment of the present invention having a separate vacuum envelope.

FIG. 4 is a detail view of a flange/scintillator assembly operable foruse in a preferred embodiment.

FIG. 5 is a cross-sectional view of a microchannel plate embodiment ofthe present invention.

DETAILED DESCRIPTION Context of the Invention

Referring now to the drawings and particularly to FIGS. 1A and 1B, thereis shown a schematic illustration of a preferred operational context ofthe instant invention. In this, borehole tool 110 is shown for analyzingmaterials from a formation 114. The tool 110 is suspended in theborehole 112 from the lower end of a cable 115. The cable 115 isanchored onto and lowered from the surface of the borehole. On thesurface, cable 115 is electrically coupled to an electronic controlsystem 118 for information storage and processing. The tool 110 includesan elongated body 119 which encloses the downhole portion of the toolcontrol system 116. The elongated body 119 may also carry a fluidadmitting channel 120 and an extendable tool anchoring member 121 whichare arranged on opposite sides of the elongated body 119. Admitted fluidflows through an analysis line 130, note FIG. 1B, and analysis cell 128.Radiation is passed through a filter 124, the admitted fluid, and areference channel of an analysis cell 128. Radiation detectors 132measure the radiation after it has traversed the analysis cell. Theinstant invention is a radiation detector that may be used to advantagein this and other contexts where significant noise and high temperatureissues are encountered.

High Temperature Radiation Detector

The structure of a first embodiment of the instant invention is shown inFIG. 2. The three major components of the radiation detector comprise ascintillating material 202, a photocathode 204, and an electronmultiplier 206. Scintillating material 202 produces light in the form ofphotons in response to incident radiation. These photons are bounced offthe wall of the detector and make their way to an open surface of thescintillating material 202. A photocathode 204 is deposited directlyonto the surface of the scintillating material 202 with no gap of air orcoupling agent disposed between. This effectively eliminates the loss ofphotons during traversal of the interface between the scintillatingmaterial 202 and the photocathode 204 which is a weakness inherent inprior art systems.

When a photon comes into contact with photocathode 204, three majorevents occur: 1) the incident photon is absorbed and its energy istransferred to an electron within the photocathode material, 2) theelectron migrates to the surface of the atom, and 3) the electronescapes from the surface of the photocathode material. The greater thenumber of incident photons, the larger the number of electrons that willbe emitted from the photocathode. Scintillating materials are carefullychosen to not react and change the operation of the highly reactivephotocathode. There may be a separate layer of non-reactive materialdeposited between scintillating material 202 and photocathode 204. Thiswould not change the overall operation of the detector and may be usedto isolate the chemically reactive photocathode from the surface of thescintillating material.

After the electrons are produced by the photocathode 204, it isnecessary to use the electron multiplier 206 to amplify the signal to ameasurable level. Electron multipliers are well known in the art and area common part of PMTs. The one shown here comprises a set of focuselectrodes 210 for directing the electrons to a dynode stack 208. Eachdynode in the stack 208 multiplies the number of incident electrons withan additive effect leading to a greatly increased signal. This amplifiedsignal is conveyed along wire 212 to a mechanism for detecting theamount of output current. This measurement is used to determine theamount and intensity of radiation encountered by the detector.

It is further necessary that scintillating material 202, photocathode204, and electron multiplier 206 are enclosed in a vacuum. It is alsopreferred to make the surface of the scintillating material that facesthe electron multiplier 206 concave with respect to the electronmultiplier. This aids in directing the maximum number of electrons tothe multiplier 206.

Proper construction of this type of radiation detector is necessary toproduce an operational system. During construction and operation, theradiation detector will be subjected to high temperatures and mustoperate at 100° C. and higher. Accordingly, the materials used as thescintillating material 202 and the photocathode 204 must be selected toexhibit compatible thermomechanical properties. It is necessary for thescintillating material 202 and photocathode 204 to expand at a similarrate so that the photocathode layer remains intact. Very fewscintillating materials are available that fit all constraints. In thepreferred embodiment, LuAP (LuAlO₃:Ce³⁺) is used. Lutetium basedcompounds are advantageous because they are slightly radioactive in thatthey produce light with no input of radiation. The decay of thisradioactivity over time is known, so these compounds may be used tocalibrate the radiation detector. Other compounds that can be used areBGO (Bi₄Ge₃O₁₂), LSO (Lu₂SiO₅:Ce³⁺), LYSO (Lu_((2x)), Y_((x))SiO₅:Ce)GSO (Gd₂SiO5); YAP (YAlO₃:Ce³⁺), and LPS (Lu₂Si₂O₇). Another class ofscintillating materials that may be used are difluorides andtrifluorides. These types of compounds have a very fast scintillationdecay of 800 ps at a wavelength of 210 nm. This short wavelengthemission is impossible to see in the prior art separatedscintillator/photocathode configurations. The information is lost in theinterface of the elements.

All of the elements of the radiation detector 132 must be containedwithin a vacuum and in FIG. 3, one embodiment is shown. The figure showsscintillating material 302, photocathode 304, and electron multiplier306 that is made up of focus electrodes 310 and a dynode stack 308. Thisassembly is fully encapsulated within a vacuum envelope 314. Theadvantage of this embodiment is the fact that the materials aresubjected to less heat in the manufacturing process and are moreshielded from the environment of the borehole.

Another preferred embodiment includes the scintillating material as anintegral part of the vacuum envelope. FIG. 4 shows an assembly 400 thatis required for this embodiment. The scintillating material 402 ispresent along with a photocathode 406. A metal flange 404 must behermetically sealed to the scintillating material. Thisscintillator/flange assembly is then fusion welded to the electronmultiplier 206 resulting in the embodiment shown in FIG. 2. Theadvantage of this embodiment is the fact that it does not require extrastabilizing mechanics that must be used in the embodiment of FIG. 3.

The preferred scintillating materials used in this embodiment are LuAP,LSO, LYSO, YAP, and LPS. The preferred metal for the flange constructionis KOVAR, a nickel, iron, and copper alloy. The makeup of KOVAR is 29%Ni, 17% Co, 53% Fe, and 1% trace elements. This metal is chosen for itsthermomechanical properties that cause it to expand at a ratesubstantially the same as the scintillating materials and thus thedetector can operate at high temperatures. Although KOVAR is preferred,other metals that exhibit thermomechanical properties similar to thescintillating material can be used.

Another embodiment, shown in FIG. 5 utilizes a number of channelmultipliers in the form of a microchannel plate (MCP). A channelmultiplier is a channel with surfaces that emit secondary electrons whenimpacted by an electron. An MCP is a cluster of thousands of channels,each one acting as an individual multiplier. Different channelconfigurations are implemented to decrease the amount of feedback thatmay affect the output of the channel.

In FIG. 5, the MCP configuration of the present invention is shown.Scintillating material 502 generates light when excited by incidentradiation. Photocathode 504 is directly deposited onto scintillatingmaterial 502 and the output electrons move to MCP 506. The outputelectrons are then conveyed along wire 508 and the generated current ismeasured.

All of these embodiments provide the stability and appropriateperformance for radiation detection in noisy, high temperatureenvironments such as in a downhole environment. The subject invention isnot intended to be limited, however, to oil patch uses but rather findssignificant advantage in any operative environment where noise and hightemperatures are likely to be encountered during radiation detectionoperations.

Method of Operation

The present invention further comprises a method for radiation detectionin a high temperature environment. The method includes providing agenerally cylindrical scintillating member with a concave surface on oneend thereof. A photocathode material that is selected to bethermomechanically compatible and chemically neutral with respect to thescintillating member is directly deposited onto the concave surface ofthe scintillating member.

In order to measure radiation the scintillating member and PMT arepositioned within an environment for radiation detection having atemperature greater than or equal to 100° C. Alternatively, thescintillating material and PMT may be lowered through zones of materialsduring well logging operations. The scintillating member detectsradiation from the high temperature environment by liberating electronsfrom the photocathode material by bombarding the photocathode withphotons from the scintillating member. The number of electrons is thenamplified through the use of an electron multiplier.

The method further comprises the step of maintaining a vacuum envelopearound the scintillating member and photocathode. This vacuum envelopecan be formed by fusion welding a flange/scintillating member assemblyto the electron multiplier. The detection is further facilitated bymatching the thermomechanical characteristics of said photocathodematerial with said scintillating material to avoid destabilization ofsaid photocathode material when said detector is placed within a hightemperature environment. In certain instances, a thin layer ofnon-reactive material is interposed between the scintillating member andphotocathode material to preserve the integrity of the photocathode.

SUMMARY OF MAJOR ADVANTAGES OF THE INVENTION

After reading and understanding the foregoing detailed description ofthe subject radiation detector for operation at high temperatures inaccordance with the preferred embodiments of the invention, it will beappreciated that several distinct advantages are obtained.

Without attempting to set forth all the desirable features of theinstant radiation detector, at least some of the major advantagesinclude providing a radiation detector for operation at hightemperatures where a photocathode material is deposited directly ontothe surface of a scintillating material for operation at hightemperatures greater than or equal to 100° C. In one embodiment, thescintillating material is an integral part of the vacuum envelope.

This structure minimizes photon loss thus increasing the sensitivity ofthe detector. The ability to operate at high temperatures is facilitatedby the selection of materials with similar thermomechanical propertiesallowing the detector to provide information in high noise andtemperature environment such as those experienced downhole in a welldrilling operation. Additionally, the proper selection of thescintillating material and the photocathode prevents reaction betweenthe highly reactive photocathode and the scintillating material.

In describing the invention, reference has been made to preferredembodiments and illustrative advantages of the invention. The subjectinvention, however, is not limited to wellhole technology and is ratherintended to provide useful application in all contexts where radiationdetection is desired. Those skilled in the art and familiar with theinstant disclosure of the subject invention may recognize additions,deletions, modifications, substitutions, and other changes which fallwithin the purview of the subject invention and claims.

1. A radiation detector for operation at high temperatures comprising: ascintillating material for emitting light when excited by incidentradiation; a photocathode deposited directly on a first surface of saidscintillating material; an electron multiplier; said first surface ofsaid scintillating material being oriented toward said electronmultiplier; said scintillating material and said photocathode comprisingcompatible thermomechanical properties such that said scintillatingmaterial and said photocathode expand at substantially similar rates;said scintillating material, said photocathode, and said electronmultiplier being sealed to form a vacuum envelope; and said hightemperatures being greater than or equal to 100° C.
 2. The radiationdetector for operation at high temperatures as defined in claim 1,wherein: said electron multiplier comprises focus electrodes and adynode stack.
 3. The radiation detector for operation at hightemperatures as defined in claim 1, wherein: said first surface of saidscintillating material is concave with respect to said electronmultiplier.
 4. The radiation detector for operation at high temperaturesas defined in claim 1, further comprising: a metal flange hermeticallysealed to said scintillating material to form a flange/scintillatorassembly.
 5. The radiation detector for operation at high temperaturesas defined in claim 4, wherein: said flange/scintillator assembly isfusion welded to said electron multiplier to form said vacuum envelope.6. The radiation detector for operation at high temperatures as definedin claim 4, wherein: said metal flange comprises a nickel, iron, andcopper alloy.
 7. (canceled)
 8. The radiation detector for operation athigh temperatures as defined in claim 1, wherein: said scintillatingmaterial is LuAP.
 9. The radiation detector for operation at hightemperatures as defined in claim 1, wherein: said scintillating materialis selected from the group consisting of: LSO, LYSO, GSO, YAP, and LPS.10. The radiation detector for operation at high temperatures as definedin claim 1, wherein: said scintillating material is a difluoride. 11.The radiation detector for operation at high temperatures as defined inclaim 1, wherein: said scintillating material is a trifluoride
 12. Theradiation detector for operation at high temperatures as defined inclaim 1, wherein: one or more layers of one or more non-reactivematerials are deposited between the scintillating material and thephotocathode. 13.-43. (canceled)